The invention relates to a fluidised-bed reactor for the production especially of rubber in the gas phase, the wall of which reactor is in the form of a cylinder in its lower portion and, adjacent thereto, is in the form of a continuously opening cone, the cone angle realtive to the centre line being from 2 to 10xc2x0 C. and the fluidized bed being higher than the cylindrical portion.
The polymerization of polyolefins in the gas phase is a process which has long been known and was first carried out on an industrial scale as early as 1968 (Ullmanns Encyklopxc3xa4die der technischen Chemie, 4th edition 1980, Vol. 19, p. 186 ff).
In that process, the actual polymerisation reaction takes place in a fluidised-bed reactor which consists of a reaction zone and, located above it, a calming zone in which the solids particles are largely separated from the gas phase. The monomers, the catalyst and possible additives, such as, for example, additives or fillers, are introduced into the reaction zone. In order to maintain a fluidised bed, a circulating gas stream is fed to the reactor from below. The circulating gas stream, which consists essentially of the unreacted monomers, is removed again at the top of the reactor, freed of residual particles, cooled and fed back into the reactor. The resulting polymer is removed from the reaction zone continuously or semi-continuously and treated further.
In the construction of apparatuses for use in the gas-phase polymerisation process, particular attention must be paid especially to tacky reaction products, in order to avoid agglutinations and caking on the reactor wall or other parts of the reactor (reactor fouling) and to avoid agglomerations of product.
That is the case especially with a fluidised-bed reactor. Accordingly, many publications deal with the construction thereof.
U.S. Pat. No. 4,003,712, for example, discloses a vertical fluidised-bed reactor which has a cylindrical lower portion followed by a short conical section and a further cylinder having a larger cross-section than the lower portion. The actual polymerisation reaction takes place in the lower portion, while in the upper portion, the calming zone, the polymer particles are separated from the stream of gas.
A large number of other publications are based on the geometry of the reactor. There are mentioned here by way of example only the following: WO 96/04322, EP-A-0 301 872, EP-B-0 475 603 and EP-A-0 728 771. The above-mentioned reactor has the disadvantage that polymer particles cake in the transition region between the lower portion and the upper portion and thus narrow the reactor cross-section continuously until the reactor must be shut down and cleaned. A common feature of all the processes is that the fluidised bed is always located in the cylindrical portion of the reactor and that sharp edges occur between the cylindrical portion and the conical sectior of the reactor. EP-A-0 765 886 discloses an apparatus for the degassing of olefin polymers, the walls of which extend at an angle of from 0.5 to 15xc2x0 C., measured with respect to the centre line.
In order to avoid so-called reactor fouling, U.S. Pat. No. 5,428,118 proposes passing a tangential stream of air over the walls of the calming zone so that deposits are avoided or deposited particles are whirled up again. Even with that teaching, the operating time of the fluidised-bed reactor in the case of rubbers can be lengthened only insignificantly.
In the publication xe2x80x9cNew Reactor in Jet Spouted Bed Regime For Catalytic Polymerizationxe2x80x9d Chem. Eng. Sci. Vol. 49, pages 4579-4588 there is further described a jet spouted bed reactor for gas-phase polymerisation which widens conically by an angle of from 10 to approximately 23xc2x0 C. immediately above the gas inlet. That reactor is said to be suitable for the handling of tacky polymer particles, the authors starting not from a fluidised-bed process but from a jet spouted bed process with plug flow. The process has not been tested on an industrial scale.
The object is, therefore, to make available a fluidised-bed reactor which does not have the disadvantages of the prior art.
According to the invention, the object is achieved by providing a fluidised-bed reactor for the production of, especially, rubbers in the gas phase, which reactor has a circular cross-section and the wall of which is in the form of a cylinder in its lower portion and, adjacent thereto, is in the form of a continuously opening cone, similar to a bell.
The fluidised bed in the reactor according to the invention extends at least over the entire length of the cylindrical portion. The fluidised bed preferably also extends into the cone, whereby the overall height of the fluidised bed should preferably not exceed 150%, especially 130%, of the length of the cylindrical portion.
The volume of the cylindrical portion is determined according to the average dwell time of the polymer particles in the fluidised bed, the height of the cylindrical portion being such that a rising, continually growing bubble fulls not more than 80% of the cross-section of the cylindrical portion. The diameter of the cylindrical portion is calculated from the volume and the height thereof.
The nominal linear velocity of the gas in the cylindrical portion must correspond at least to the rate of loosening of the polymer particles. However, the nominal linear gas velocity is preferably at least 3 times, especially from 5 to 7 times, the rate of loosening. The nominal linear gas velocity is preferably to be from 0.3 to 2 m/s, especially from 0.4 to 1 m/s, very especially from 0.5 to 0.8 m/s.
The cone angle, relative to the centre line, is preferably from 2 to 10xc2x0 C., especially from 3 to 6xc2x0 C. and very especailly from 3.5 to 5.5xc2x0 C.
The length of the cone is such that the cross-section of the reactor at the end of the cone is so great, and the nominal linear gas velocity is thus so low, that catalyst or polymer particles having a size of from 10 to 1500 xcexcm, preferably from 50 to 300 xcexcm, especially from 50 to 150 xcexcm, are no longer discharged from the reactor. By that measure, the discharge of solids from the reactor according to the invention is reduced and gumming up of the apparatuses located downstream is reduced.
The ratio of the diameter of the cylinder to the height of the reactor is preferably 1:8-15, especially 1:11-12.
The ratio of the height of the cylinder to the height of the cone is preferably 1:1.5-4, especially 1:2-2.5.
In a further preferred embodiment, the ratio of the height of the cylinder to the height of the cone is 1:80-100, so that virtually the entire casing of the reactor is in the form of a continuously opening cone.
The reactor is preferably closed off at the top by means of a hemisphere. The hemishpere may have baffles. A tangential stream of air preferably acts on the sphere, so that the hemishpere functions as a cyclone built into the reactor. The base of the reactor may be of any desired form, the form of a hemisphere being preferred.
The gas mixture to be polymerised is preferably introduced into the reactor at the lower end through a perforated gas distributor. The gas distributor must be so designed that good solids mixing prevails in the fluidised bed and gas passes over the walls of the cylindrical portion continuously in order to avoid polymer particles being deposited thereon. A perforated base is preferably used.
If necessary, a tangential stream of gas may also act upon the walls of the cone, so that no polymer deposits form.
The reactor is preferably operated at a temperature of from 20 to 160xc2x0 C. and a pressure of from 1 to 20 bar absolute. In the process according to the invention, the temperature may also be so selected that it is below the dew point of at least one constituent of the circulating gas.
The reaction is preferably manufactured from stainless steel or black steel.
The fluidised-bed reactor according to the invention is suitable for the production of polymers of any kind, preferably for the production of rubbers of any kind in the gas phase.
Within the scope of the invention, polymers are, for example, poly-xcex1-olefins, polyisoprene, polystyrene, styrene-butadiene rubber (SBR), isobutylene-isopropylene rubber (IIR), polyisobutene, polychloroprene, silicones and copolymers of ethylene and one or more of the following compounds: acrylonitrile, malonic acid ester, vinyl acetate, acrylic and methacrylic acid esters, xcex1-olefins, dienes and trienes.
Rubbers within the scope of the invention are uncrosslinked but crosslinkable polymers which can be converted into the rubber-elastic state by the crosslinking.
Preferably, however, the fluidized-bed reactor according to the invention is used for the production of ethylene-propylene copolymers (EPM), ethylene-propylene-diene terpolymers (EPDM), SBR, nitrile-butadiene rubber (NBR), polyisoprene and butyl rubber (BR) in the gas phase.
The polymers may contain the conventional additives. In the process according to the invention, the polymers may also be produced in the presence of inert fillers. Preferred fillers are carbon black, silica, clay, talcum and/or ground polyolefin.
The rubbers produced in the gas phase are suitable for the production of moulded bodies of any kind. Preferably, however, the rubbers are used for the production of motor vehicle tires.
The rubbers produced in the fluidised-bed reactor according to the invention are also preferably used in admixture with other plastics for the modification thereof.
It must be regarded as completely surprising and unexpected to the person skilled in the art that, with the reactor according to the invention, the operating times can be increased by a factor of 10 as compared with reactors of the prior art. Because the fluidsed bed extends into the cone, particles constantly flow around the transition between the cone and the cylindrical portion of the reactor, so that no deposits form especially in that sensitive area.